Ranks are Recruit, Soldier, Disciple, Warrior, Veteran, Master, Assassin/Templar. In order from lowest to highest.
Recruit -> (Special Mission) -> Soldier -> (10 EXP ) -> Disciple -> (20 EXP) -> Warrior -> (40 EXP) -> Veteran -> (60 EXP) -> Master -> (Special Mission) -> Assassin/Templar
Low level mission. Follow, information gather, delivery. etc. (1 exp)
Medium level mission. Assassinate NPC, protect npcs. etc. (2 exp)
High level mission. Alliances Encounter Missions, Assassination Members. etc. (3 exp)
For every Special Mission you do to gain a higher rank you may do a time skip.
EX: like in recruit you could be like 17, when promoted to soldier, make your char like 26.